In this chapter, we offer an overview of some of the major trends in theory development and use in relation to teaching mathematics with digital technology. We showcase some of the developments that have occurred since the first edition of this book (2014). We also provide a deep review of the multiple ways in which the instrumental approach has evolved over time, as a way to exemplify how theory development responds to new questions and new theoretical insights. Throughout the chapter, we make explicit the philosophical assumptions on which these theories depend—particularly the binaries they reify—and use these to open up consideration of different assumptions and how they might matter to our field of research.